J un 1 99 8 Radiative Transfer Modelling of the Accretion Flow onto a Star - Forming Core in W 51

We present an analysis of the temperature, density, and velocity of the molecular gas in the star-forming core around W51 e2. A previous paper (Ho & Young 1996) describes the kinematic evidence which implies that the core around e2 is contracting onto a young massive star. The current paper presents a technique for modelling the three-dimensional structure of the core by simulating spectral line images of the source and comparing those images to observed data. The primary conclusions of this work are that the molecular gas in e2 is radially contracting at about 5 km s and that the temperature and density of the gas decrease outward over 0.15 pc scales. The simple model of the collapse of the singular isothermal sphere for low-mass star formation (Shu 1977) is an inadequate description of this high-mass molecular core; better models have temperature ∝ r, density ∝ r, and velocity ∝ r. The core appears to be spherical rather than disk-like at the scale of these observations, 0.3 pc. In this paper we show how a series of models of gradually increasing complexity can be used to investigate the sensitivity of the model to its parameters. Major sources of uncertainty for this method and this dataset are the interdependence of temperature and density, the assumed NH3 abundance, the distance uncertainty, and flux calibration of the data. Subject headings: ISM: individual(W51)— ISM:kinematics and dyamics— ISM:molecules—stars:formation